In a typical process of producing acrylonitrile by reacting: propylene and/or propane; ammonia; and oxygen in the presence of a catalyst, firstly, a reaction-generated gas containing generated acrylonitrile, acetonitrile, and hydrogen cyanide is cooled in a quench column, and unreacted ammonia is removed by neutralization with sulfuric acid. Thereafter, the reaction-generated gas is fed to an absorption column to allow acrylonitrile, acetonitrile, and hydrogen cyanide to be absorbed in water. Subsequently, an aqueous solution obtained in the absorption column and containing acrylonitrile and the like is introduced in a collection column, and the aqueous solution is separated into a fraction containing acetonitrile and most of water and a fraction containing most of acrylonitrile and hydrogen cyanide by distillation operation. Thereafter, the fraction containing most of acrylonitrile and hydrogen cyanide is introduced to a hydrocyanic acid-removing dehydration column to separate hydrogen cyanide and water from the fraction, and the bottom liquid is then introduced to a product column to purify acrylonitrile by distillation operation, thereby obtaining a product that conforms to a product specification.
Patent Literature 1 discloses a method for suppressing polymerization of acrylonitrile and hydrogen cyanide in purification of acrylonitrile by adding an acid and hydroquinone in the hydrocyanic acid-removing dehydration column.